An articulated road vehicle is a vehicle which has a permanent or semi-permanent pivoting joint in its construction, allowing the vehicle to turn more sharply. There are many types of articulated road vehicles and a most common type is the semi-trailer truck. In the semi-trailer truck, a trailer without a front axle, commonly referred to as a “semi-trailer”, is coupled to a “prime mover”, also commonly referred to as a truck or tractor.
The semi-trailers are substantially longer in length than the prime mover and provide a load-carrying tray or other payload means such as tank for liquids. The tray is generally supported by a plurality of a non-steerable (rigid) tandem axles located at the rear end of the semi-trailer. A turntable is provided at the front end of the semi-trailer for coupling with the prime mover. The turntable, sometimes termed a “fifth wheel”, allows the trailer to articulate about a substantially vertical axis relative to the prime mover. This enables the semi-trailer to articulate thereby enabling the semi-trailer to turn around corners and otherwise manoeuvre along road networks.
Semi trailer trucks typically have a semi-trailer maximum length of about 53 feet in the US, about 43 feet in Australia, and a slightly shorter maximum length in Europe. Due to the length of such semi-trailer trucks, it is difficult to carry out tight turns and manoeuvre in tight quarters, as is the case in many city and town intersections. The ability of such a semi-trailer truck to effect such turns is commonly indicated by the term “swept path performance”. Sometimes these trucks will be crossing at least two lanes of the road to effect the turn. This is because the path followed by the wheels of the prime mover will be quite different to the path followed by the “non-steerable axle” wheels of the trailer in a phenomenon known as “off tracking”. This results in what is known as “tyre scrubbing” where the tyres of the trailer axles are simultaneously rolled and dragged over the road surface. This leads to premature wearing of the tread portion of the tyres of the trailer wheels. With reference to FIG. 1, when such a prior art semi-trailer truck 12 with a prime mover 13 negotiates a turn, wheels 10 on rigid tandem axles of semi-trailer 14, have a tendency to move in the straight ahead direction shown by arrow S. This not only can cause problems with tyre wear, but affects the stability of the vehicle and safety to the driver and others in the surrounding environment, as the vehicle may be prone to “rolling” as it enters the turn. The stability problem is even more pronounced when the vehicle is fully loaded and the centre of gravity is higher, as for example is the case for a fully laden fuel tanker. Also the tendency of wheels 10 to want to move in the straight direction S, can in addition to tyre wear cause serious damage to roads.
Various attempts have been made to overcome the problem associated with the prior art, and some of these attempts are described in the “prior art” section of U.S. Pat. No. 7,219,913 (Atley), which is itself directed to an arrangement to improve the cornering characteristics (swept path performance) of an articulated vehicle. Many of the prior art arrangements are complex systems including mechanical and hydraulic components, which attempt to steer wheels on the trailer, and some require a prime mover which is significantly different to a conventional prime mover, making such arrangements costly and complex. While the arrangement in U.S. Pat. No. 7,219,913 comes some way to improve over the prior art, it is still a complex system which employs a steering mechanism on the trailer to steer wheels on a sub-chassis of the trailer.
A further prior art arrangement employing a driven steer carriage for use on a trailer is described in U.S. Pat. No. 7,726,423 (Atley). This arrangement utilises a power source to drive a pair of carriage wheels via a drive transmission. Whilst such arrangement may significantly improve the “swept path performance” of an articulated vehicle, it not only is a complex system, but also a costly one.
The present invention seeks to overcome at least some of the abovementioned disadvantages by providing a self-steering bogie for a road vehicle.